As you know, defects in the welding technique can negatively affect the quality of welded material. Various methods are involved in welding a material. In this article, you will learn about the full penetration welding technique, also known as the complete penetration weld.
What is a full penetration weld? A full penetration welding technique involves joining the two pieces of metal with no gaps in between. In this type of weld, the filler material fills the joint’s roots. This technique is mostly applied to metal products with high-stress resistance.
The full penetration method results in higher strength as compared to partial penetration. As a result, metal products with full penetration are ideal for metal products designed for high-stress applications.
A complete or full penetration welding results in a thick weld. The weld is beveled and penetrated with different weld passes. However, in joints with 0.20 inches or larger thickness, penetration is achieved without requiring a groove bevel.
A full or complete penetration weld can be a thick weld that has been beveled and then penetrated by being filled with multiple welds passes, or it could be a sheet metal joint that is .020″ thick and is penetrated without any need for a groove bevel.
Deep penetration requires very high power densities of more than 1 megawatt per square meter. The resultant beam results in improved welding and the stronger metal product.
In a full penetration weld, the metal pieces are fused using butt weld that fuses completely with the base metal. The strength of the base metal is the same across the joints. This full penetration joint can be in different configurations including the single- and double-vee configurations.
If you had done partial penetration welding, the fused metal will have a lower strength resistance factor as compared to the base metal.
How is a full penetration weld is achieved?
Different factors affect the quality of complete penetration welding. Let’s take a look at some of the factors that affect weld quality.
1. Welding current
Welding current is the foremost factor that affects the penetration of the weld. A higher weld current will result in deeper penetration. The opposite is the case with a lower weld current.
You can change the welding current by adjusting the speed of the wire feed. In other words, the desired current can be achieved by varying the distance between the tip and the metal.
There is an opposite relationship between the tip-to-metal distance and the welding current. If you increase the distance, the current will decrease. This will result in partial penetration. In contrast, if you decrease the tip-to-metal distance, the current will increase which will lead to full penetration welding.
By varying the tip-to-metal distance, you can control the current applied to weld the metal pieces. Note that you should not weld at a particular distance for long periods in certain cases. Consider varying the distance a bit to prevent burn through in case there are discontinuities in the joint gap.
2. Arc stability
Arc stability is another factor that affects full weld penetration. The arc must be stable for deep penetration. Arc travel speed directly affects the extent of penetration.
You can achieve greater penetration with higher arc travel speed. The penetration will be full at an arc speed of 12-inches per minute (ipm) or 30.5 centimeters per minute (cpm). At 7 ipm or 17.8 cpm, the penetration will decrease.
The penetration level will also decrease if the arc travel speed exceeds 17 ipm or 43.2 ipm. With higher speeds, the heat generated from welding does not have enough time to melt the base metal area. In the case of lower arc speed, a lot of filler material is deposited in the area. The liquid weld roll in the front and prevents deeper penetration.
3. Torch position
The position of the welding torch also affects the extent of penetration. But the effect is slightly less as compared to arc travel speed or welding current.
The backhand welding techniques generally result in deeper penetration as compared to the forehand welding technique. You can achieve maximum penetration using the backhand welding and torch angle of 25 degrees. But a higher torch angle results in arc instability that prevents deeper penetration.
4. Deposition rate
The deposition rate is related to the wire feed-speed. You can change the current and the tip-to-metal distance to achieve a particular deposition rate.
A small tip-to-metal distance will result in low wire-feed speed and high welding current. The result will be a low deposition rate and deeper penetration. The penetration will be higher since less metal is deposited at a given current.
To achieve shallower penetration, a long tip-to-metal distance is suitable. This will results in a low deposition rate at a given current resulting in less penetration weld. The high weld metal deposited will result in cushioning of the arc force that prevents deeper penetration.
What are the benefits of full penetration welding?
Full penetration weld has several benefits as compared to partial or low penetration weld.
1. Corrosion resistance
Metals that are welded using full penetration weld have a higher resistance to corrosion. A full penetration weld results in a fused joint line. In contrast, the joint line is not filled with partial penetration. The space between the joints results in potential differentials that result in corrosion of the metal.
2. Stronger weld
As mentioned previously, full penetration weld results in a much stronger weld as compared to partial penetration weld. The joint is filled in full penetration weld. This results in an extra volume of the metal resulting in higher stress resistance.
In contrast, the welds with partial penetration techniques have gaps in the joint line. The gaps concentrate the geometric stress.
3. Complete fusion
Full penetration weld covers more area. Broader penetration will increase the chances that the filler will reach the root resulting in complete fusion. In the case of partial penetration of the weld and the resultant narrower penetration profile, there is a chance of incomplete fusion at the metal’s root.
Achieving a complete fusion at the weld joint root is critical. In case the tip-to-metal distance is long, and the electrode is not properly aimed at the root, the lack of fusion can create structural issues of the resultant metal product.
Are there any cons of full penetration welding?
While full penetration results in stronger metal, there are certain cons of the technique that you should know.
1. Weld puddle
The first problem with full penetration weld is that it can result in too much admixture at the base metal in some situations. Higher penetration results in an increased volume of the base metal. An increase in the base metal volume can result in a weld puddle that increases the chances of a crack.
Examples, where deep penetration weld is not suitable, include welding on metals with a higher level of lead, phosphorus, or sulfur. These elements have lower melting temperatures as compared to steel. This results in a weld puddle that decreases the quality of the welded metal. The puddle may transfer to the weld’s center that can lead to cracking at the center.
2. Dilution of the weld
In overlay applications and hard facing, deep penetration results in diluted weld deposits. This results in a lower wear resistance profile of the welded metal.
Overlay welds with maximum penetration result in increased admixture of the base plate and the welding metal. The opposite is the case with partial penetration resulting in better quality metal products.
3. Burn through
Deep penetration weld is also not suitable when welding on think material. Too much penetration on a thin sheet of metal will result in the weld to burn through the metal joint.
Full penetration is also not suitable when a root pass has to be made in pipe joint or open root joints. Too much penetration can result in the weld to burn through the root thereby decreasing the structural integrity of the material.
4. Centerline cracking
Lastly, deep penetration can also result in centerline cracking. To avoid centerline or hot cracking, there should be a balance between the penetration level and the root pass width. The ration between the penetration depth and width should not be more than 1:1.2. A higher ratio will result in irregular weld shape creating reduced resistance to stresses.
Conclusion
Full penetration involves fusing the entire between the weld joints. You should use the full penetration welds appropriately. The technique is preferable to partial penetration to ensure high strength material with greater corrosion and stress resistance.
You should be aware of the cons of deep penetration such as susceptibility to burn through, cracking, wed puddle, and diluted weld deposits in certain cases. The information in this blog will help you to use the right welding methods for a project.